Posted
by
timothy
on Monday June 13, 2005 @06:42PM
from the ice-is-free-after-all dept.

inkey string writes "Summer has arrived, and I've been busy slowly overheating in my student house without central air.
I decided to put my thermodynamics classes to work however, and produced this ~24$ homebrew air conditioner. It'll cool a room to a comfortable level in 15-20 mins, and will run for a few hours on a garbage pail full of water.
It's cheap, environmentally friendly (just fire the waste water off to your garden), and makes a good one hour project for a quiet evening."

From the article: "cheap, environmentally friendly".
From who.int: "Billions without clean water": link [who.int]
The guy has no clue how lucky he is in his "student ghettos don't have gardens" home to have clean water to throw around.

But sadly this isnt that revolutionary, nor is it very 'green'. It takes a cold source of water to work, and if you have none in your area (tap water wont cut it unless you happen to get fed from a pipe running through a glacier) you have to get cold media from your local refridgerator/freezer. Why not instead rig a direct cycle through your cooling appliance of choice to offer a small, localized cooling effect? It also wouldn't waste water. Just remember, don't try to cool the room with the freezer in it.

Exactly, I think the financial folks talk about trading off initial costs for long term running costs. Someone is footing the bill on the electricity for making that ice. BTW the overall energy efficiency of this setup is substantially lower than the equivalent decent window air conditioner (COP of the air conditioner is much higher than the COP of an ice maker due to smaller delta T iirc).

#1 you can buy a bag of ice at the gas station/convenience store, not free but then neither is the electricity to run your freezer.

#2 even if you used the house freezer, you shut the door and basically you're pumping heat away from the bedroom into the kitchen, obviously you won't get huge temperature differentials, but 5-6C feels very noticeable when you're trying to fall asleep and it's too hot to do so.

Why you can get the ice out of the freezer/refrigerator in your kitchen. It just costs a little electricity to make. It makes it by using a condensing coil to use a gas medium to pump heat from the freezer are to coils on the back of the fridge which... then heats the room you are trying to cool. DOH!

While I agree that there are far more elegant ways to do this, You can still cool a room this way and not disobey the laws of Thermodynamics.

The heat generated by the Fridge stays in the Kitchen. Close the door and now you have effectively transfered heat from the cool room (bedroom or livingroom) to the kitchen. It is now far easier to relax.

Catching the water and freezing it is a bad idea. Again, it's thermodynamics. The heat the water has picked up has to go somewhere when you freeze it. If we're talking about your kitchen refrigerator here, that heat is dumped back into the room, PLUS the energy used by the fridge to move the heat out of the water. Net result: the room gets hotter.

In order for this to work you can't have your room as a closed system. If it is laws of thermodynamics say no matter what you do it will only get hotter. You have to dump the waste water out or bring cold water in to get any cooler.

If you had a freezer outside, then you could pump the water to a bucket outside, move that into the freezer and bring in a chilled bucket to continue cooling the room. But honestly, who has a working freezer sitting in their backyard?

Many people think that if you leave the refrigerator open, it will cool down the house. However, all a refrigerator does is take heat from its inside and move it to the outside. That's why those coils in the back are warm. Thus, if you open up the fridge to cool the house down, all you'll do is make the place warmer (since the efficiency isn't 100%). Also, you'll probably burn out the motor in your refrigerator.

I'll grant that in your particular comment, putting a fan in front of the freezer would work for a while, because it's already cold. But it wouldn't work for long.

I'd say it's more environmentally friendly than a freon-based system, or other gas-based coolers. True, it still requires electricity for cooling the water and running the fan, but there's no real "waste" material from the whole device.

Uhh yeah, except for the fact that he's using his freon-based refrigerator to make the ice, which then ventilates its heat exaust into the same room that he is attempting to cool off using his jerry rigged system.

look.. using a freezer(that you run in the same area as you're trying to cool) to cool a room isn't that smart.

I thought the guy had built up some swamp cooler or something like that, as that does really work - but this is just friggin lame/stupid in the long run to use. now... he could refine the idea quite a bit - use ground cooling to keep the water cool for example(just bury some copper pipes or some container a meter into the ground).

"hey i built a cooler. well, i just brought some ice water to my room!!!".

If he's getting ice from the freezer in his apartment/room, it is negating his attempt to cool his apartment/room. Heat removed from water in the freezer to make the water freeze is put right back into the house by the thermal coils on the back of freezer/refridgerator.

#1 you can buy a bag of ice at the gas station/convenience store, not free but then neither is the electricity to run your freezer.

The store needs to make a profit on top of the cost of the electricity to maintain the machine, and the ice...

...supplied by the ice company which bought the machine, maintains it, and freezes the ice, and trucks it to the store from their "plant"...and make a profit.

You do realize that 1kW/hr costs about 22 cents, whereas a 20lb bag of ice costs about $5, right?

You have to move 330J of energy to freeze one gram of water, basically. We'll assume a 50% efficiency here (pretty poor, I believe). A bag of ice, say, 20lb- would need about 3 million joules (watt-seconds), or 6 million watt-seconds of electricity. That's 1662 Watt-hours, roughly.

Or about 36 cents.

#2 even if you used the house freezer, you shut the door and basically you're pumping heat away from the bedroom into the kitchen, obviously you won't get huge temperature differentials

Most refrigerators are virtually incapable of pumping that much heat (there's a reason they're insulated), and furthermore, are designed to work at a temperature range 60-90 degrees cooler than what you're asking of it. Ever noticed that a fridge takes forever to get from room temperature down to operating temperature?

This idea is so stupid, I can't believe I just wasted 5 minutes on this post. I want that 5 minutes of my life back.

Actually, the prices Californians pay for power offsets the cost to deliver electricity to customers in the Northwest. I'm reminded of the slogan at Chilkoot Charlie's in Anchorage: "We cheat the other guys and pass the savings along to you."

I'm quite upset with you. No self respecting Canadian, or any member of any country other than the lagging-behind-the-times-USA uses non-metric measurements. Canada and fahrenheit should never be mentioned in the same sentence, unless it's one of the following:

"That Fahrenheight 911 show was pretty good eh?"

"Fahrenheight? It's too damn hot in here for your jibberish. Go fill up the $24 AC with ice so we can get the temperature to a respectable level of Centrigadey goodness."

Hate to be a buzz kill, but I really take issue with using clean tap water as a resource for no more than its specific heat and the fact that it happens to be cool. And then dumping that tap water on the ground? That water is not free; your public services dam it, filter it, treat it and pipe it to you, sans cryptosporidium you know.

This is an unconscionable waste of resources. I'd be so impressed if you found a way to store the warm water to boil your pasta or at least wash your laundry.

The amount of heat that this increased load will generate is exactly the amount that the ice will be capable of cooling off. Actually that 'exactly' is wrong, it is even more than the ice will be capable of cooling because of mechanical inefficiencies in the freezer itself.

Cold water from fridge is used to run through piping/fan to cool room. It's not done by swamp or other methods. The only thing going on is the warm air is blown past the tube of cool water, bringing the temperature of the room down, and the temperature of the water up. (The water doesn't go through any phase changes through the tubing or anything; it's simply equalizing the temperature)

Result from Step 2:

Heat n is transferred from the air to the water.

Room is warm and so is the water once again around room temp (going out the window now).

Heat m is added from the inefficiency of the fan.

(x + z - n) + (y - z + n ) + i + m

Net Result: (original heat)+ (excess heat)

x + y + i + m

We started with x + y. Now we have x + y + i + m.Seeing the problem yet?

The water going through the tubing is *not* superheated. It's not warmer than the room air. At the very best it's the same temp as the room. That's if he gets complete transfer. No net heat is removed. It's added.

Water y gets dumped out the window. What are you left with? x ++

Please show your math for your strange theory that makes this perpetual motion machine work, and show how the water in the tube is well above room temperature in order to decrease the net temperature of the room.

As others have said, this setup has all sorts of problems, from a reliance upon a source of ice that may very well be dumping more heat into the local environment than it saves, to wasting water.

Very good point. Just where is the fridge located that produces the ice? Where is it putting the heat. The fridge is not included in the price of the project. Why not just take the door off the fridge and mount the fridge in the wall to expell the heat elswhere?

For those who don't know, fridges use a small compressor because they are cooling a small insulated enclosed space. They do not provide enough cooling to deal with the heat influx of a large room. It's BTU capacity is way undersized.

His fridge would not produce enough ice to keep his cooler supplied. The ice is a overnight creation cold storage medium to provide a short coling burst. This is not a cooling solution.

Air conditioning compressors displacement is designed to be effecient at expected cold side pressures and high (hot) side pressures. A fridge compressor is sized to work with lower suction pressures (larger displacement) for the creation of Ice in the freezer compartment. Running it with constantly elevated tempratures will overload the compressor causing ineffeciency.

When buying a compressor, they are sized for high temp use (air conditioning) and low temprature (freezers) The diffrence is the displacement is sized to the expected low side pressure.

High pressure moves more BTU/watt. Low pressure is for a large temprature differential.

With all respect to university resource limitations, you would think they would be most proud of having their students attracting world attention, especially for something laudable.

Web pages for professors hardly get any news coverage, and these people are supposed to be at the top of the game. Surely research funding would leap to another quantum level if professors discussed on their websites how much impact or influence their research has, especially if those sites attracted page hits from large numbers of the public.

The adage of publish or perish seems to become perish and perish (i.e., lose, lose) when it comes to a little slashdotting.